Exercise Won’t Selectively Activate the VMO but Electric Stimulation Will

Effects of electrical stimulation of vastus medialis obliquus muscle in patients with patellofemoral pain syndrome: an electromyographic analysis. Garcia FR1, Azevedo FM, Alves N, Carvalho AC, Padovani CR, Negrão Filho RF. Rev Bras Fisioter. 2010 Nov-Dec;14(6):477-82.

The use of surface electromyography (SEMG) has been considered a tool for quantitative assessment of patellofemoral pain syndrome (PFPS). Conservative treatments aim to improve patellar alignment, and electrical stimulation of the vastus medialis obliquus (VMO) muscle has been considered effective because it is selective and does not cause joint irritation.

This study aims to investigate the efficiency of a muscle strengthening program with electrical stimulation of the VMO muscle in PFPS by SEMG.

A group of ten young women (age: 23.1 ± 4.9 years; body mass: 66.8 ± 14.0 kg; height: 1.63 ± 6.9 cm; BMI: 25.1 ± 5.6 kg/m²) with unilateral PFPS participated in the study. They performed the functional test of stair stepping to capture the electromyographic (EMG) activity of the VMO and vastus lateralis (VL) muscles, before and after a program of electrical stimulation of the VMO muscle. The electrical stimulation was performed three times per week for six weeks. For an analysis between the VMO and VL muscles, we considered the variables: ratio of time of onset to peak of activation, ratio of the integrals of the signals (t-test for dependent samples), and any difference between onsets of activation (Wilcoxon test), with a significance level of p < 0.05.

The results only showed  change in behavior in the EMG signal for the ratio of the integrals of the signals, indicating that changes occurred in the force-generating capacity of the muscle after the training.

The use of electrical stimulation should be considered to complement the conservative therapeutic approach in patients with PFPS, and the analysis of the ratio of the integrals of the SEMG signals should be considered as an instrument of evaluation.

My comments:
I’m not a fan of attempting to selectively strengthen the VMO (oblique fibers of the vastus medialis) for patellofemoral pain. The idea is that by doing so, it will help the patella track more medially and thus decreasing stress on lateral structures. The reason I’m not a fan is due to reading study after study, there is an indication that it’s pretty much impossible to preferentially recruit the VMO with exercise. As a physical therapist I hear about it all the time, and though the VMO might help pull the knee medial, when you do exercise the brain activates all aspects of the quads, lateral as well as medial. Exercises attempting to tweak the balance towards the medial by turning the toes out, coactivating the hip adductors, etc. just don’t pull it off. If any exercise does make a percent of difference between VM or VMO vs. vastus lateralis (VL) activation ratios, they generally do so by making the entire quadriceps EMG decrease. That said, electrical muscle stimulation (EMS) is different. You put EMS electrodes on a portion of a large muscle like the quadriceps and that portion of the muscle will contract to the exclusion of the rest. It’s plainly evident if you try it on yourself, which is what I think every physical therapist should do.

This study tested the EMS theory out to see if doing so would get the VMO to activate earlier, which it did not. However, it did make the VMO activate stronger after training with EMS 3 times per week for 6 weeks. Such that the electromyographic signal ratio of the VMO/VL increased from 0.89 to 1.82, which is pretty substantial. Whether this helps to decrease patellofemoral pain is another matter that was not tested, as it appears that the primary muscle weakness seen with patellofemoral pain isn’t quadriceps, but rather at the hip. Still it makes me think that if I’m going to put some number of EMS electrodes on the quadriceps, at least one of them is going to be on the vastus medialis. I was doing that most of the time already, but until I have a better idea, I figure I’ll do it all the time now.
EMS parameters used in this study were as follows:

  • Waveform: asymmetric bipolar
  • Frequency: 50 Hz
  • Pulse duration: 500 US
  • Treatment duration: 7 minutes
  • Duty Cycle and treatment duration: 6 sec on 12 sec off for 7 minutes, progressing to 10 sec on 20 sec off for 30 minutes.
  • mA: “maximum intensity the participant could bear, without pain during contraction”

Those are not  the parameters I would use but they seemed pretty effective anyway. One of the downsides of EMS research is that parameters used are all over the place, with certain guidelines emerging as good, but still lots of exceptions. The bright side is that more often than not, the exceptions from the norm still work.

Thanks for reading my blog. If you have any questions or comments (even hostile ones) please don’t hesitate to ask/share. If you’re reading one of my older blogs, perhaps unrelated to neck or back pain, and it helps you, please remember Spinal Flow Yoga for you or someone you know in the future.

Chad Reilly is a Physical Therapist, obtaining his Master’s in Physical Therapy from Northern Arizona University. He graduated Summa Cum Laude with a B.S. Exercise Science also from NAU. He is a Certified Strength and Conditioning Specialist, and holds a USA Weightlifting Club Coach Certification as well as a NASM Personal Training Certificate. Chad completed his Yoga Teacher Training at Sampoorna Yoga in Goa, India.

6 thoughts on “Exercise Won’t Selectively Activate the VMO but Electric Stimulation Will”

  1. Hi Chad,

    Do you have any advice on how to place electrodes to target the vmo? I have read it’s important to find the muscle’s motor points but I suspect the vmo in particular has weakened and I’d like to try strengthening it selectively to the extent that this is possible. PT has not resolved my knee pain and my glute/hip strength is as good as we can get it. ems is sort of a last resort I’m ready to try.

    Thanks for any advice or resources!

    • Hi AJ,

      I have a guy right now had a patellar subluxation but was CrossFitter and who I got to do RDLs with upwards of 180 kg. Stronger than his squats, so hip extension wasn’t a problem. He worked up to maxing out my hip abduction machine, but was weaker at the start so it’s how to say how much of a given factor contributes. He’s doing well though and we’re doing relatively isolated EMS to VM/VMO.

      Anyway, we tried a few EMS pad placements and what he liked best was one electrode strapped right over his vastus medialis close to the knee, that I expect hit all the oblique fibers along with the rest of his VM. The other electrode we placed about 10-12 inches higher (more proximal), but also on the medial aspect of his thigh. Motor points are real, but I don’t think are anything you want to worry about if you have a strong EMS machine, programmed well, with large rubber carbon electrodes. My go to electrodes are about 3.5″ circular. With the sticky electrodes that come with a lot of machines it’s sometimes too painful turn the machine up enough to get full benefit. More details here. I wrote some more thoughts on motor points here. I hope that helps!

    • Hi Ravi,

      Electric stimulation (VMO) us just electric stimulation, as I do it, but with one electrode placed directly over the VMO (vastus medialis oblique) muscle fibers. Thus directly activating that portion of the muscle. That said, I still don’t think the VMO is more important than any muscle, in fact I think it’s less since it’s not even a whole muscle in and of itself. Thanks, for your question, I hope that helps.

    • Rupert,

      Sorry for getting back to you late. I’m not sure how much I have blogged about it, but for PFPS, strengthening the hips and hamstring muscles seems to be very important. What’s new that might help is I created Spinal Flow Yoga for neck and back pain, but it is pretty much a total body workout at this point, with a strong emphasis on hips and hamstring strength. So it’s something that takes only 5 minutes a day. Also the high rep lunges, on several people I know has made crackling knee joints from the PF joints stop cracking. I think the high reps smooth out the joint. It’s anecdotal but only $20 to try and see if it works for you.


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